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21.
A variety of processes and sources account for the total accumulation of sediment on a fringe marsh. The rates of accretion across Brockenberry fringe marsh at the south end of the Delmarva Peninsula, Virginia, were determined by Pb-210 radiogeochronology. Rates are governed by the surface elevation with respect to midtide elevation, the rate of sea-level rise, and outwash from the mainland. Only some portions of the fringe marsh are able to keep pace with sea-level rise and thus migrate up the mainland slope during transgression. 相似文献
22.
Since their introduction to the geological literature by Brace (1959, 1960, 1961), Mohr circles for large irrotational deformations have proved valuable as aids to our understanding of deformation geometry. However, confusion persists regarding sign conventions. We show that there are two basic kinds of Mohr circles, each with its distinct set of sign conventions. These two divisions, which we call Mohr circles of the First and Second Kind, are not merely reflections of one another in Mohr space. They represent two distinct aspects of the relationship between the space of tensor components (Mohr space) and the space of geological structures (geographical space). The distinction between Mohr circles of the First and Second Kind is critical when the circles are drawn in off-axis positions for asymmetric tensors. Constructions in Mohr space are described which correspond to various standard tensor operations including transposition, inversion, addition and various kinds of multiplication. For some of these operations Mohr circles of one kind or the other offer advantages. 相似文献
23.
This study provides a single-point position estimation technique for interplanetary missions by observing visible planets using star trackers. Closed-form least-squares solution is obtained by minimizing the sum of the expected object-space squared distance errors. A weighted least-squares solution is provided by an iterative procedure. The weights are evaluated using the distances to the planets estimated by the least-squares solution. It is shown that the weighted approach only requires one iteration to converge and results in significant accuracy gains compared to simple least squares approach. The light-time correction is taken into account while the star-light aberration cannot be implemented in single-point estimation as it requires knowledge of the observer velocity. The proposed method is numerically validated through a statistical scenario as follows. A three-dimensional grid of test cases is generated: two dimensions sweep through the ecliptic plane and the third dimension sweeps through time from January 1, 2018 to January 1, 2043 in 5-year increments. The observer position is estimated at each test case and the estimate error is recorded. The results obtained show that a large majority of positions are well suited to position estimation by using star trackers pointing to visible planets, and reliable and accurate single-point position estimations can be provided in interplanetary missions. The proposed approach is suitable to be used to initiate a filtering technique to increase the estimation accuracy. 相似文献
24.
An improved age framework for late Quaternary silicic eruptions in northern Central America 总被引:1,自引:0,他引:1
William I. Rose F. Michael Conway Carlos R. Pullinger Alan Deino William C. McIntosh 《Bulletin of Volcanology》1999,61(1-2):106-120
Five new stepwise-heating 40Ar/39Ar ages and one new high-sensitivity 14C date of ash-fall and ash-flow deposits from late Quaternary silicic volcanoes in northern Central America document the eruption
rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango)
located north of the basalt, andesite, and dacite stratovolcanoes of the Central American volcanic front. These deposits form
extensive time-stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valuable
framework for age determinations of more localized volcanic and nonvolcanic events. The new data, especially when integrated
with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite
a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because
of their locations near the highly vulnerable economic hearts of Guatemala and El Salvador, the risks of eruptions from these
calderas should be carefully considered along with risks of major earthquakes and volcanic front volcanoes, which are much
more frequent but inflict less severe and extensive damage. This investigation also includes some examples of dating efforts
that failed to produce reasonable results.
Received: 15 May 1998 / Accepted: 18 January 1999 相似文献
25.
Hurford G.J. Schmahl E.J. Schwartz R.A. Conway A.J. Aschwanden M.J. Csillaghy A. Dennis B.R. Johns-Krull C. Krucker S. Lin R.P. McTiernan J. Metcalf T.R. Sato J. Smith D.M. 《Solar physics》2002,210(1-2):61-86
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to
17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation
collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons,
ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to
convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware,
the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms.
Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The
characteristic strengths and weaknesses of this type of imaging are summarized. 相似文献
26.
Aschwanden Markus J. Metcalf Thomas R. Krucker Säm Sato Jun Conway Andrew J. Hurford G.J. Schmahl Edward J. 《Solar physics》2004,219(1):149-157
We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High-Energy Solar Spectroscopic
Imager (RHESSI) spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy
ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM,
MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and
spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field of view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics
are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence
behavior and a photometric accuracy in the order of a few percent, while MEM does not converge optimally for large degrees
of freedom (for large field of view and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This
comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/B:SOLA.0000021801.83038.aa 相似文献
27.
New results from a 1 Gyr integration of the martian orbit are presented along with a seasonally resolved energy balance climate model employed to illuminate the gross characteristics of the long-term atmospheric pressure evolution. We present a new analysis of the statistical variation of the martian obliquity and precession prior to and subsequent to the formation of the Tharsis uplift, and explore the long term effects on the martian climate. We find that seasonal polar cycles have a critical influence on the ability for the regolith to release CO2 at high obliquities, and find that the atmospheric CO2 actually decreases at high obliquities due to the cooling effect of polar deposits at latitudes where seasonal caps form. At low obliquity, the formation of massive, permanent polar caps depends critically on the values of the frost albedo, Afrost, and frost emissivity, ?frost. Using our model with values of Afrost=0.67 and ?frost=0.55, matched to the NASA Ames General Circulation Model (GCM) results (Haberle et al., 1993, J. Geophys. Res. 98, 3093-3123, and Haberle et al., 2003, Icarus 161, 66-89), we find that permanent caps only form at low obliquities (<13°), suggesting that any permanent deposits on the surface of Mars today may be residuals left over from a period of very low obliquity, or are the result of mechanisms not represented by this model. Thus, contrary to expectations, the martian atmospheric pressure is remarkable static over time, and decreases both at high and low obliquity. Also, from our one billion year orbital model, we present new results on the fraction of time Mars is expected to experience periods of low obliquity and high obliquity. 相似文献
28.
R.R. Conway 《Planetary and Space Science》1983,31(10):1223-1228
Modeling of the dayglow spectrum in the wavelength region between 3300 and 3500 Å indicates that the N2 second positive (0,0) band at 3371 Å is blended with the Vegard-Kaplan (0,9) band. A recent analysis of rocket observations of the dayglow shows that 20–30% of a 3371 Å narrow band filter-photometer signal is due to the VK emission (Conway and Christensen, 1983). Kopp (1977) and Hernandez (1983) reported analyses of 3371 Å photometer observations from the Visible Airglow Experiment on the Atmospheric Explorer-C (AE-C) satellite which did not consider the Vegard-Kaplan (VK) emission. The observations were compared to theoretical estimates of the second positive volume emission rate based on a photoelectron model and on absolute fluxes measured by the Photoelectron Spectrometer experiments on AE-C and AE-E. Inclusion of the VK band in the AE analysis would bring the reported photoelectron theory into agreement with the airglow observations. However, the overestimate of the N2 second positive airglow predicted by the AE-E photoelectron flux measurements increases to a factor of nearly two rather than the 20–30% reported by Hernandez (1983). 相似文献
29.
The position angle of linear polarization and the degree of circular polarization of the decimetric emission show no marked variations with time, although there may be an indication in the position angle data of a secular decrease in the inclination of the magnetic field structure. High resolution observations of the radiation belts show that the asymmetry in the emission found in synthesis maps originates from an extended region. The presence of this features accounts well for the observed variation in the position of the centroid of the emission with rotation. The radio observations suggest that any equatorial displacement of the main magnetic field structure from the center of the planet is less than 0.05 RJ. 相似文献
30.